Stepped surface diffuser

ABSTRACT

An optical device such as an overlay panel for a LCD display, comprises an element of light-transmitting material having a surface configured to form a stepped Fresnel prismatic structure. The light-transmitting material itself or ribbed light refracting element, said element incorporates an array of graded refractive index features adapted to impart light dispersing of diffusing characteristics to the light-transmitting material. In an alternative arrangement, the element of light-transmitting material has a layer configured to form a stepped Fresnel-type surface and an additional layer incorporating such an array of graded refractive index features.

BACKGROUND OF THE INVENTION

The present invention relates to a light diffusing sheet material havingutility in various fields, including that of enhancing video displays,such as LCD displays.

In liquid crystal displays the bright image seen by the viewer isgenerated through the liquid cell either by light generated within theassembly by a back light or by the use of ambient light which is firsttransmitted through the cell and reflected at the rear of the cell,re-emerging through the cell to create a bright image. In this latterarrangement, due to the position of the viewer, the majority of lightmust be accepted off axis, preferably should not be further diffused onentering the display, and should preferably re-emerge on axis with someadditional diffusion in order to provide an acceptable viewing cone. Itis well known that Fresnel-like structures have the ability to redirectlight; as circular arrangements of facets to create lenses, or as lineararrangements of facets to create off axis effects. Structures of thesetypes have been proposed incorporating materials with light diffusingcharacteristics, as described in U.S. Pat. No. 4,911,529, where thediffusive effect is provided by a so-called bulk diffuser comprisingsmall particles of a material of a first refractive index dispersed in amatrix material of a second refractive index. Such materials aredescribed in, for example U.S. Pat. No. 4,983,016, EP-A-0464499 orEP-A-0843203. An important characteristic of such bulk diff-usermaterials is that the amount of diffusion taking place within a givencomposition is dependent on the thickness of material through which thelight travels. As a result, light entering an assembly containing suchmaterial off axis becomes more diffuse and reduced in intensity thanlight which enters on axis.

SUMMARY OF THE INVENTION

According to the present invention there is provided an optical devicecomprising an element of light-transmitting material having a surfacethereof configured to form a stepped, Fresnel-type light refractingelement, such light transmitting material incorporating an array ofintegral graded refractive index structures adapted to impart lightdispersing or diff-using characteristics to said light-transmittingsheet material or the element having a layer configured to form saidstepped surface and an additional layer incorporating such an array ofintegral graded refractive index structures.

In certain instances, it may be desirable to render one surface of saidelement reflecting or partially reflecting by coating with a materialwith the necessary optical properties, such as a metal applied bydeposition or sputtering.

The sheet material may comprise, for example, a photopolymer, in whichthe graded refractive index features are formed by exposure of aprecursor of the material, (such as a monomer) to appropriate radiationin a predetermined pattern so as to produce localised variations oflight intensity within the material and hence localised variations indegree of polymerisation and thus in refractive index. As anotherexample, the material may be dichromated gelatine (DCG), in which thedesired graded refractive index features are formed by exposure of thematerial itself to such a radiation pattern, to produce an equivalenteffect. Such materials, correctly processed, possess the previouslydescribed desirable optical characteristics. The process techniques usedmay, for example include, those disclosed in EP-0294122 or EP-0801767 orU.S. Pat. No. 5,695,895 or GB-A-1499135. These types of diffusingmaterials may themselves have off axis or asymmetric characteristics, asdescribed in EP 0768565, which can further enhance the off axis effectsgenerated by the Fresnel structure. Methods of producing sheets of lightdiffusing material in which the light-diffusing characteristics are dueto refractive index variations within the material are known and willnot be discussed in greater detail here. These known methods, however,are directed to the production of either planar light-diffusing sheetsor light-diffusing sheets having a surface configuration in the form ofa plurality of convex domes, for example, adapted to enhance thediffusive characteristics of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below with reference to theaccompanying drawings, in which:

FIG. 1 is a view, in section perpendicular to the plane of majorextension of the material, of a first embodiment of the invention,

FIGS. 1a and 1 b are corresponding views of variants each having areflective coating,

FIG. 2 is a view, partly in section and partly in perspective of anotherform of light-diffusing sheet in accordance with the invention,

FIGS. 2a and 3, 3 a and 3 b are sectional views similar to FIG. 1,showing specific examples of materials in accordance with the invention,

FIG. 4 is a graph showing diffusive characteristics for variousmaterials, and

FIGS. 5a to 5 h are diagrams similar to FIG. 1 showing various differentribbed profiles which may be adopted in embodiments of the invention.

In the embodiments of the present invention shown in the drawings, thedevice has the general form of a sheet 13 of light diffusing material,one surface of which is provided with a stepped or groovedconfiguration, after the fashion of a Fresnel lens or prism. As aresult, the device, in addition to its diffusive characteristics, alsohas a directional characteristic in the sense that, if a parallel beamof light is directed on the sheet from one side thereof, the polardistribution of the diffused light emerging from the device has a peakalong a direction which differs from the direction of the axis of theincident beam by an angle representative of the deviation imparted bythe prismatic component due to the Fresnel prism surface configuration.The stepped surface of the device has, in cross-section, as shown in theFigures, a saw-tooth profile section, comprising major facets or rampsinclined typically at 10° to 20° to the general plane of the sheet, andminor facets extending generally perpendicular to said plane.

DETAILED DESCRIPTION OF THE INVENTION

In the example shown in FIG. 1, a light beam entering the material frombelow, normal to the (planar) lower surface is shown as passing throughthe material undeviated to pass through one of the facets of the Fresnelstructure forming the upper surface, to be refracted through a 5° angle.In the example shown, the Fresnel structure comprises a series of majorfacets each inclined at 10° to the plane of the lower surface. It will,of course, be understood that these figures are merely exemplary. Itwill also be understood that the illustration of FIG. 1 neglects thediffusive effect of the material. Taking this effect into account, itwill be appreciated that the overall effect, for a parallel beamentering the material along the ray path indicated, is to produce,within the material, a spreading or scattering of light in a polardistribution with an intensity peal, along the ray path shown.

Referring to FIG. 1b, an arrangement is shown which is similar to thatof FIG. 1 except that the stepped upper surface is rendered reflective,for example by metallisation e.g. applied by vapour deposition or bysputtering. The incorporation of a reflecting surface removed from thesurface through which the light enters the assembly, causes the light,initially entering the material from below at a significant (35°-40°)angle to the normal, to be reflected at the reflective facets, passingthrough the material a second time to become more diffuse beforeemerging substantially on axis, that is to say emerging as a dispersing“bundle” of rays in a distribution having a maximum along the normal tothe planar face. In some embodiments, the reflecting surface may be onlypartially light reflecting and may be partially light transmitting,instead of being fully light reflecting. Such a “transflective” materialcan be useful eg. with displays which can be alternatively front-lit orback-lit.

FIG. 1a shows an arrangement similar to that of FIG. 1b, except that thelight transmitting layer provided with the Fresnel-faceted surface (andwhich layer is indicated at 22 in FIG. 1a) is of non-diffusive,transparent material and the desired diffusive effect is provided by anadditional layer, referenced 23, of light-diffusing material. In theembodiments of FIGS. 1a and 1 b described above, in which reflectingcoating is used, that coating is applied to the corrugated or facetedsurface of the material, arranged as the rear surface of the screen, inorder to secure a more pronounced off-axis effect. However, thereflective coating may, of course, be applied to the flat surface of thedevice, again arranged as the rear surface.

FIG. 2 shows, partly in section and partly in perspective, alight-diffusing, light-transmissive sheet in accordance with theinvention, comprising a layer 32 having a Fresnel stepped or facetedsurface, and a flat substrate layer 33. The layer 32 may be oflight-diffusing material and substrate 33 of transparent non-diffusivematerial, such as polyester or polycarbonate, or, similarly to thearrangement of FIG. 1a, the layer 32 may be non-diffusive and the layer33 diffusive.

FIG. 2a shows a specific example of an arrangement corresponding to thatof FIG. 2 wherein the Fresnel structure has a pitch of 50 microns and afacet angle of 10 degrees. Again, neglecting, initially, the diffusivecharacteristics of the material, a light beam entering on axis (i.e.normal to the “plane” of the material), exits at 5° to the normal.

FIG. 3 shows an embodiment similar to that of FIG. 1 and having, by wayof example, a Fresnel structure the pitch of which is 50 microns with afacet angle of 20 degrees. FIG. 3a shows a structure of the same pitchand facet angle, and shows additionally, for illustration, different raypaths through the light diffusing material 13. FIG. 3b shows a structuresimilar to that of FIG. 2a, but in which, however, the substrate layer,referenced 43, is a layer incorporating graded refractive index featuresarranged to impart a net deflective effect upon light passingtherethrough as well as, typically, diffusing the light passingtherethrough. Thus, in the example illustrated, light entering the layer43 normal to its exposed lower (in the figure) surface, is subjected toa net, or average deflection of 20°. The Fresnel-faceted layerreferenced 42, in this case may simply be transparent and non-diffusiveor may itself be light-diffusing.

The ray diagrams in FIGS. 1 to 3 b, as noted, neglect, for ease ofillustration, the fact that the light-transmitting layers of the deviceare not simply transparent, but that one or each such layer has alight-diffusing character.

Thus, in place of the emerging ray illustrated, there will, in practice,be a dispersing “bundle” of rays in a distribution having a maximumalong the emerging ray path illustrated.

The stepped surface in the above embodiments may define, in effect, aplurality of precisely parallel similar V-section grooves extendingacross the sheet material so that, neglecting the diffusive features,the material acts as a tin prism. Alternatively, however, the groovesand ridges defined by the stepped surface may extend in circles or arcsand be of a form corresponding to that of the stepped surface of aFresnel lens, whereby the refraction or reflection at the steppedsurface will tend to “focus” the diffusive light provided thereby. Inaddition, the presence of the diffusing material will mask thediscontinuities in the Fresnel structure to the viewer.

In the preferred embodiments discussed, the light diffusing character ofthe light diffusing layer arises as a result of incorporating an arrayof graded refractive index features, for example graded refractive indexmicrolenses or other features. These graded refractive index microlensespreferably have each a transverse dimension, i.e. a dimension measuredparallel with the major planes of the sheet, which is small in relationto the pitch of the corrugated surface, i.e. the spacing between, forexample, peaks of adjacent ridges of the corrugated surface, the gradedrefractive index features being likewise closely spaced in relation toone another, so that, for example, the mean spacing between adjacentsuch features may be several orders of magnitude less than the pitch ofthe corrugations on the surface. In the examples discussed, the gradedrefractive index features typically have an average diameter of 5microns spaced apart (centre to centre), by, for example, a distance of8 microns. In each said graded refractive index feature, the refractiveindex may be substantially constant along any line perpendicular to theplane of the sheet but may vary with transverse position in the sheetmaterial. Thus, in the case of a graded refractive index lens, therefractive index may vary with radial distance from the optical axis ofthe lens, as described in EP-A-0294122. In variants, however, the gradedrefractive index diffuser may have features aligned along axes whichpass through the sheet material obliquely, so that whilst the refractiveindex may be substantially constant along any line parallel with suchaxes, and may vary with transverse position in a plane perpendicular tothese axes, that plane will no longer be parallel with the plane of thematerial, and where the GRIN features are GRIN lenses, the principalaxes of the lenses will be inclined to the perpendicular to the plane ofthe material. Such a variant graded refractive index diffuser may ingeneral exhibit an off-axis diffusion characteristic of its own

Whilst in FIGS. 1a to 3 b, the ribs or grooves formed on the surface ofthe material are shown as a series of identical ribs of identicalsaw-tooth profile, the applicants have found that in some applicationsit can be advantageous to vary the rib profile over the sheet material.Thus, for example, as shown in FIGS. 5a to 5 h, a wide variety ofprofiles may be adopted. For example, as shown in FIG. 5a, alternateribs in the series may have major flanks of different inclinations α andβ with respect to the general plane of die base surface of the sheetmaterial, (herein referred to as the base plane), for example 8 degreesand 14 degrees, alternately. The crests of the ribs may be of the sameheight or of different heights, as shown in FIG. 5g. Again, as shown inFIG. 5h, where rib faces of two different inclinations α and β areprovided, these need not be provided on different ribs but may beprovided as sections of different inclinations on the same ribs asshown. Similarly, each of a series of identical ribs, for example, mayeach have two, three or more portions of the same rib surface withdifferent inclinations α,β and δ with respect to the base plane, cf.FIG. 5e and FIG. 5h. Whilst the ribs may be of generally saw-toothprofile with one side inclined significantly to the base plane and theother side more or less perpendicular to the base plane,(eg. inclined at2 to 5 degrees to the perpendicular to the base plane), in someapplications the two sides of each rib may be more or less equallyinclined to the base plane, for example, as shown in FIGS. 5b and 5 c.Again, as illustrated in FIG. 5f, one,(or each), flank of each ofselected ribs may be concavely curved, (see FIG. 5f), or convexlycurved, (not shown). These measures make it possible to secure a productwith a two-lobed or three-lobed diffusion characteristics, or withotherwise modified diffusion polar distributions.

The refractive index gradations in the light-diffusing material may beproduced by photographic means, for example, by contact printing throughan appropriate mask as described in EP-A-0294122. However, whereas inEP-A-0294122, for example, the optical mask utilised is preferablyplane, for the purposes of the present invention, the optical mask maycomprise, for example, a glass plate having the desired steppedconfiguration on one surface and having that one surface coated with athin layer of, for example, metallic chrome, so thin as not to alter thecorrugated character of the glass surface, the chrome layer beingprovided with an array of transparent windows or apertures etchedtherein by photo-etching techniques so as to form an optical printingmask. The stepped, chromed surface of the plate may then be pressedagainst the exposed surface of, for example, a layer ofphotopolymerisable monomer (destined to form the layer 33 of FIG. 2, forexample) on a transparent substrate, so as to form the desiredcorrugations by what is effectively an embossing technique.Alternatively the photopolymerisable material may be cast on the steppedchromed surface. The layer of photopolymerisable monomer, still incontact with the glass plate or mask, may then be exposed topolymerising light through such mask. Thereafter, and after a subsequentblanket exposure to polymerising light, for example directed through thetransparent substrate, the plate may be separated from the finishedproduct. Alternatively, the diffuser/Fresnel lens or prism combinationcan be created a to step process; forming a planar diffuser aspreviously described then embossing the Fresnel structure into onesurface using a combination of heat and some pressure. In this instancethe embossing may be effected using a copper die, formed by machiningthe Fresnel structure into the surface of a copper plate using a diamondtool. Whilst the above description, and EP-A-0294122, make reference tothe use of photopolymer as the material in which graded refractive indexfeatures are induced by exposure to appropriate light or otherradiation, other materials in which refractive index variations cansimilarly be induced by such exposure, may be used utilising the sameexposure techniques. Thus, for example, dichromated gelatine (DCG) maybe used as the material in which graded refractive index lenses or othergraded refractive index features are induced to impart light diffusingcharacteristics to the device.

It is preferable, particularly where the devices are to be used inconjunction with LCD displays, that the device in accordance with theinvention or the several layers thereof, should be non-birefringent,i.e. should be polarisation-maintaining.

A particular use of a material in accordance with the invention is as anoverlap for front-lit LCD displays in, for example, portable telephones,portable computers etc., to allow the user to view the display from theoptimum angle without obstructing incident light and without beingtroubled by extraneous surface reflections. In such an application ofcourse, no reflective coating is utilised and the or each layer of thediffusing screen should be light-transmitting. In an arrangement such asillustrated in FIG. 2, the substrate may be constituted by the, forexample glass, cover plate of the LCD display.

Of equal significance are devices in accordance with the invention inwhich, as in FIGS. 1a and 1 b above, one surface, usually the surface inwhich the Fresnel structure is present, is coated with a reflecting orpartially reflecting coating, typically a metal such as aluminum.Devices of this type may be incorporated behind the liquid crystal cellin a display illuminated using ambient lighting.

In the arrangements of FIGS. 1a and 1 b, light is diffused to a minimumextent on arrival at 35 to 40 degrees to the normal to the lower surface(i.e. the normal to the general “plane” of the material) but is diffusedmore strongly after reflection at the reflective coating, during thereturn passage through the diffusing material. These differences indiffusion and intensity are clearly shown in FIG. 4 where intensity andangle or view resulting from different light entry angles are compared.Thus, FIG. 4 shows the characteristics of a planar sheet of gradedrefractive index light diffusing material. Graph A was derived bydirecting a beam of light through such sheet of material, arrangedperpendicular to the beam axis and measuring the intensity of light(plotted along the Y-axis) emerging from the sheet on the opposite sidethereof along directions at various angles (plotted along the X axis)from the beam axis. Graph B was produced in the same way but with thesheet of graded refractive index diffusing material arranged with thenormal to its plane angled at 35° with respect to the beam axis.

Whilst, in some of the embodiments described above, the gradedrefractive index features responsible for the diffusion of light areformed integrally with the material affording the stepped or facetedsurface, in other embodiments within the scope of the invention thedevice is formed as a plurality (e.g. two, three or more) of distinct,superimposed layers, with one such layer, preferably. an outer layer,being of a transparent material of uniform refractive index but providedwith the ramped or faceted refractive surface and another such layer,for example juxtaposed with the layer with the faceted surface,incorporating the giraded refractive index features.

In embodiments such as shown in FIGS. 2, 2 a, 3, 3 a, and 3 b in whichlight may pass entirely through the stepped or ribbed surface, ratherthan being reflected at that surface, the stepped surface of thematerial 13, 32, or 42 may be covered by a layer of a transparentmaterial with a refractive index significantly different from that ofthe material 13, 32 or 42, the covering material intimately conformingto the stepped or ribbed surface, without air-gaps and thus completelyfilling the grooves defined between adjacent steps or ribs, the coveringmaterial further providing, as its surface remote from the stepped orribbed surface, a smooth planar surface of the resulting device whichwill, for example, make it easier to keep free from dust and dirt andfacilitate lamination with other planar surfaces, eg. of parts of LCD orother displays. A similar technique for providing a device having planarsurfaces whilst retaining optical effects due to predetermined surfaceconfigurations of light transmitting layers is disclosed inGB-A-2314943, to which reference should be had.

What is claimed is:
 1. An LCD display having a cover plate with anoverlay comprising an element of light-transmitting material having asurface thereof configured to form a stepped or ribbed light refractingand/or reflecting element, said element of light-transmitting materialincorporating an array of graded refractive index features adapted toimpart light dispersing or diffusing characteristics to saidlight-transmitting material, or the element of light-transmittingmaterial having a layer configured to form said stepped or ribbedsurface and an additional layer incorporating said array of gradedrefractive index features.
 2. The LCD display according to claim 1,wherein the stepped or ribbed surface of said element oflight-transmitting material is covered with a second material of arefractive index significantly different from that of thefirst-mentioned light transmitting material, the second materialproviding a smooth, generally planar surface.
 3. The LCD displayaccording to claim 1, wherein said graded refractive index features areof a size which is small in relation to the spacing between adjacentsteps or ribs of the stepped or ribbed surface of said element oflight-transmitting material.
 4. The LCD display according to claim 1,wherein said element of light-transmitting material has the general formof an extended, generally planar sheet or layer, and said gradedrefractive index features each have an axis extending through saidplanar sheet or layer from one side of said generally planar sheet orlayer to the other, each said graded refractive index feature having agraded refractive index distribution, in a plane perpendicular to saidaxis, which is substantially the same in different such parallel planesat successive respective positions along said axis.
 5. The LCD displayaccording to claim 4, wherein each said axis extends perpendicular tothe plane of said planar sheet or layer.
 6. The LCD display according toclaim 4, wherein at least some said axes are inclined with respect tothe plane of said planar sheet or layer.
 7. The LCD display according toclaim 4, wherein each said graded refractive index features is a lenshaving an optical axis corresponding to the first-mentioned axisthereof, the refractive index within said structure being substantiallyconstant along a line parallel with said axis and varying with distancefrom said axis.
 8. The LCD display according to claim 4, in which thestepped or ribbed surface is covered with a second material of arefractive index significantly different from that of thefirst-mentioned light transmitting material, the second materialproviding a smooth, generally planar surface.
 9. The LCD displayaccording to claim 1, wherein, on said stepped or ribbed surface, thesteps or ribs are of saw-tooth form.
 10. The LCD display according toclaim 9, in which the corresponding surfaces of adjacent steps or ribsare generally parallel so that the stepped surface is a Fresnel-typesurface.
 11. An LCD display including a liquid crystal cell and, behindthe liquid crystal cell, an element of light-transmitting materialhaving a surface thereof configured to form a stepped or ribbed lightrefracting and/or reflecting element, said element of light-transmittingmaterial incorporating an array of graded refractive index featuresadapted to impart light dispersing or diffusing characteristics to saidlight-transmitting material, or the element of light-transmittingmaterial having a layer configured to form said stepped or ribbedsurface and an additional layer incorporating said array of gradedrefractive index features, wherein the surface of saidlight-transmitting material furthest from the liquid crystal cell iscoated with light-reflecting material.
 12. The LCD display according toclaim 11, wherein the coating of light-reflecting material is such thatthe coated surface is partly light reflecting and partly lighttransmitting.
 13. The LCD display according to claim 12, wherein thestepped or ribbed surface of said element of light-transmitting materialis covered with a second material of a refractive index significantlydifferent from that of the first-mentioned light transmitting material,the second material providing a smooth, generally planar surface. 14.The LCD display according to claim 11, wherein the stepped surface iscoated with light-reflecting material.
 15. The LCD display according toclaim 11, wherein the stepped or ribbed surface of said element oflight-transmitting material is covered with a second material of arefractive index significantly different from that of thefirst-mentioned light transmitting material, the second materialproviding a smooth, generally planar surface.
 16. The LCD displayaccording to claim 11, wherein said graded refractive index features areof a size which is small in relation to the spacing between adjacentsteps or ribs of the stepped or ribbed surface of said element oflight-transmitting material.
 17. The LCD display according to claim 11,wherein said element of light-transmitting material has the general formof an extended, generally planar sheet or layer, and said gradedrefractive index features each have an axis extending through saidplanar sheet or layer from one side of said generally planar sheet orlayer to the other, each said graded refractive index feature having agraded refractive index distribution, in a plane perpendicular to saidaxis, which is substantially the same in different such parallel planesat successive respective positions along said axis.
 18. The LCD displayaccording to claim 11, wherein, on said stepped or ribbed surface, thesteps or ribs are of saw-tooth form.
 19. The LCD display according toclaim 18, in which the corresponding surfaces of adjacent steps or ribsare generally parallel so that the stepped surface is a Fresnel-typesurface.